This project represents the completion of an extramural grant that I was awarded just before coming to NIEHS. It is a case-control study of residential magnetic fields and breast cancer nested within a cohort of African-Americans, Latinos and Caucasians in Los Angeles County. Data collection is complete and I have spend the past year analyzing the data. We have data for 738 incident cases and 692 controls. The manuscript is nearly complete and will be submitted by the end of FY 2002. The hypothesis that magnetic fields influence breast cancer risk is based on the existence of a plausible mechanism related to the effect of magnetic fields on melatonin along with some limited epidemiologic data. The study has two specific goals. 1. To determine if residential exposure to magnetic fields, as assessed by wiring configuration coding and direct measurements is associated with the risk of breast cancer. 2. A secondary hypothesis is that particular combinations of the alternating current (AC) magnetic field and the direct current (DC) magnetic field, increase the risk of breast cancer. We had previously conducted a case-control study of magnetic field exposure in relation to leukemia risk in Los Angeles County. We were able to apply methods developed for that study to the breast cancer study. We have completed a draft manuscript for this study which is being circulated to all co-authors and will be submitted shortly. The study is unique in being the only study of magnetic fields and cancer risk in the US which for which the base population is completely defined. Because we have based this study in an existing cohort, we have residential address information on all potential subjects. We were able to obtain an indirect measure of magnetic field exposure, wiring configuration, on virtually all eligible subjects. Thus this primary analysis is free from selection bias, a limitation of previous case-control studies. Another advantage of this study is that the subjects are predominantly members of two understudied minority groups - African-Americans and Latinos. Related to this work, we have analyzed data from a previous study that we conducted on health effects of magnetic fields. It had been proposed that the associations seen between magnetic fields and leukemia risk might be due to a true effect of traffic related air pollution. We found that a weak association between traffic density and leukemia was due to confounding by the magnetic field association. Thus traffic density, as a surrogate for air pollution, does not explain the magnetic field effects we observed in Los Angeles.